1. Technical Field
The present invention relates to a method of sintering a porous glass base material that includes performing sintering, dehydration, and transparent vitrification on the porous glass base material that will be the raw material for an optical fiber.
2. Related Art
A porous glass base material that serves as the raw material for an optical fiber is manufactured by using vapor axial deposition (VAD) or outside vapor deposition (OVD) to deposit glass fine particles on a starting member. The porous glass base material is hung from the top of a long shaft formed by jointed members made of Si3N4, for example, and inserted into a furnace tube that includes an environment of an inert gas and a gas for a dehydration reaction. The porous glass base material is then lowered vertically from above while being rotated and passed through a heated region to undergo the sintering, dehydration, and transparent vitrification processes, thereby obtaining an optical fiber preform formed by a core and cladding. After elongating this preform to have a prescribed diameter, the preform is drawn by a drawing mechanism to obtain the optical fiber.
During this sintering process performed while the porous glass base material is hung from the tip of a long jointed shaft, if an earthquake or large vibration (referred to hereinafter simply as an earthquake) occurs, the shaft and base material swing around violently and this load causes damage to weak portions such as the joints of the shaft or the portion where the shaft connects to the base material, thereby making it easy for the base material to fall. When the base material falls, not only does the base material become unusable, but it also damages the furnace tube and the sintering apparatus.
Particularly in recent years, the size of porous glass base materials has increased rapidly in an attempt to decrease costs, and this greatly increases the risk of falling. In order to prevent such falling due to earthquakes, methods have been proposed for providing a seismic isolator to prevent shaking, but this proposal significantly increases the equipment cost and creates space problems when providing this seismic isolator in existing equipment.
It is an objective of the present invention to provide a method for sintering a porous glass base material that solves the above problems without decreasing the quality when performing sintering, dehydration, and transparent vitrification on the porous glass base material.